Sodium azide is a base-pair substitution mutagen that is widely used as a positive control chemical for Salmonella typhimurium TA-100 and TA-1535. It was found that when petri dishes containing sodium azide were placed in a 37 degrees C incubator, a volatile mutagen was produced which elevated mutagenic responses on other petri dishes. This volatile mutagen was produced at concentrations as low as 1 microgram sodium azide per plate.